In recent years, a solar cell has been widely employed as a device directly converting solar energy into electric energy from the viewpoints of effective use of natural resources and prevention of environmental pollution. Further developments of solar cells are in progress with the object of power generation efficiency and durability.
As shown in FIG. 1, for example, a structure of the solar cell is known in which a light receiving side transparent protection material 11 such as a glass substrate, a light receiving side sealing film 13A, a photovoltaic elements 14 such as crystalline silicon cells, a backside sealing film 13B and a backside protection material (back cover) 12 are laminated in this order and are adhesively combined each other.
In order to generate a large electrical output, a solar cell has plural photovoltaic elements 14 connected to each other by using a connecting tab(s) 15. Therefore, photovoltaic elements are sealed by using insulating sealing films 13A, 13B so as to maintain insulation property of photovoltaic elements 14.
In the related art, as a sealing film for such solar cells, a film made of ethylene-polar monomer copolymer such as ethylene-vinyl acetate copolymer (occasionally abbreviated to EVA) or ethylene-ethyl acrylate copolymer (EEA) is used. Especially, the EVA film is preferably used because it has low cost and high transparency. Furthermore, in the EVA film used for the sealing film, a crosslinker such as an organic peroxide other than EVA is used to improve crosslink density and thus the durability and strength of the film are improved.
Moreover, in recent years, several sheets have been developed such as a solar cell sealant seat containing EVA and ethylene-(meth)acrylic acid-acrylic acid copolymer for improving transparency and insulation property (Patent Literature 1), and a sealing film composition or a sealing sheet containing an ethylene base copolymer such as EVA and a polyolefin such as polyethylene for improving heat resistance, creep resistance and steam permeation resistance (Patent Literatures 2 and 3).
Meanwhile, when producing the solar cell by using a sealing film which contains EVA, in general, the light receiving side transparent protection material, light receiving side sealing film, photovoltaic elements, backside sealing film and backside protection material are laminated in this order, are heated and are pressed by a laminator so as to crosslink and cure EVA and finally they are adhesively combined each other.